The following two patents are cited for showing diffractive lens elements, or lenslets. In U.S. Pat. No. 3,547,546, issued Dec. 15, 1970, entitled "Multiple Image Forming Device", H. Schier describes a zone plate matrix 10 having a plurality of zone plates 12, each being a diffractory optical image forming device. Each zone plate 12 is formed from a series of concentric annular zones 14 and 16 of alternating transparent and non-transparent regions which decrease in width from a center outward, or from all transparent zones (FIG. 4), in which alternate zones shift the phase of radiation by 180.degree. with respect to adjacent zones. In U.S. Pat. No. 4,530,736, issued Jul. 23, 1985, entitled "Method for Manufacturing Fresnel Phase Reversal Plate Lenses", W. E. Mutter discloses a photolithographic process for fabricating phase reversal plate and sinusoidal phase reversal plate Fresnel phase plate lenses. p As seen in FIG. 1a, a conventional wavefront sensor is designed to achieve a null result with a flat input wavefront. In FIG. 1a, a flat wavefront (FWF) impinges on an optical element comprised of a two dimensional array 1 of diffractive lenslets 1A. Each of the lenslets 1A of the array 1, shown in the plan view of FIG. 1c, focuses the incident FWF onto a surface of a sensor 2. The sensor 2 may be a Charge Coupled Device (CCD) having a two dimensional array of radiation sensors for sensing the focussed radiation.
The system of FIG. 1a is suitable for applications in which a null result is desired, that is, a flat input wavefront is provided. However, for those applications in which a measurement of wavefront aberration (i.e., aberration within the dynamic range of the sensor) is desired, a sensor that nulls with a FWF is not as accurate as a sensor that nulls when a desired aberrated wavefront (AWF) is input.
As seen in FIG. 1b, if the FWF is replaced by an AWF, the focal positions of the lenslets 1A of the array 1 are shifted by an amount that is related to a magnitude of the aberration at each lenslet 1A.
Referring to FIG. 2, one application for measuring very large aberration is encountered when testing fast aspheric mirrors at the center of curvature. A special set of optics, referred to as a null corrector 3, is positioned in the optical path. The null corrector 3 is designed, in accordance with an expected wavefront aberration, so as to convert the AWF to a FWF. Assuming that the null corrector 3 is properly designed, when an AWF having the expected aberration is incident on the null corrector 3, the lenslets 1A of the array 1 each focus the radiation onto predetermined ones of the sensors 2, giving the result of FIG. 1a. However, if the actual aberration of the incident wavefront differs from the expected aberration, the null corrector 3 will not operate as intended, and the result will more closely resemble FIG. 1b.
As can be appreciated, one disadvantage of the system shown in FIG. 2 is the requirement to design, align, and maintain the separate null corrector 3.
It is thus an object of this invention to provide a wavefront sensor having a lenslet array that also functions as a null corrector.
It is another object of the invention to provide a wavefront sensor having a lenslet array that also functions as a null corrector, with each lenslet being designed so as to counter an expected local wavefront tilt with an equal and opposite tilt.